Time delays are ubiquitous in nature. They occur in a wide variety of natural and man-made control systems. In an environment with a networked control system (NCS), a physical or virtual device may provide sensor feedback to a controller, which in turn controls the output or operation of the devices via control instructions. Both types of communications may be sent over a communications network. Communications between the device and the controller (both sensing and control communications) are sometimes delayed as a result of the nature of the communications network (e.g., the network uses a slower transmission medium), problems or technical difficulties in the network (e.g., router failure), or the activities of an attacker intentionally attempting to degrade the performance of the network.
Time delays in the sensing and control communications can impact the stability of a system and degrade its performance, for example, when sensor telemetry messages from the device arrive with such a significant delay that it is difficult for the controller to appropriately react and adjust the operations of the device.
In power systems, these types of time delays exist in the sensing and control loops. A “traditional” controller of power systems is designed based on current information being available and ignores time delays even if they are present. However, power grids are constantly being enhanced with new telecommunication technologies for monitoring in order to improve efficiency, reliability, and sustainability of supply and distribution. For example, the introduction of a wide area measurement system (WAMS) provides synchronized near real-time measurements in phase measurement units (PMUs). WAMS can be used for stability analysis of power systems and can be used for efficient controller design. Nevertheless, time delays are present in PMUs measurements as a result of natural transmission lines [D. Dotta, A. S. Silva and I. C. Decker, “Wide-area measurements-based two-level control design considering signal transmission delay”, IEEE Trans. on Power Systems, vol. 24, no. 1, 2009].
Furthermore, modern power grids rely on computers and multi-purpose networks, making them vulnerable to cyber-attacks that can cause a major threat to life and economic productivity. A time-delay-switch attack (TDS) is a type of cyber attack where an adversary chooses to introduce delays into an NCS. Thus, it is important to investigate methods of attack on industrial control systems and devise countermeasures and security control protocols that can react to them.